The Transcriptional And Translational Regulation Of HanA During Heterocyst Development In Anabaena Sp. PCC 7120

Anabaena sp. PCC 7120 is a filamentous cyanobacterium which can carry out oxygenic photosynthsis and nitrogen fixation simultaneously. When the growth medium is deprived of combined nitrogen sources,5%-10% of the cells along each filament will differentiate into heterocysts which are specialized for nitrogen fixation. During the course of heterocyst differentiation, the expression of many genes changes dramatically.The expression of a histone-like protein, HanA, in the process of heterocyst differentiation in Anabaena sp. PCC 7120 was investigated in this study. HU exists as a homodimer in Anabaena sp. PCC 7120. HU proteins are critical to maintenance of the nucleoid structure. In addition, they participate in all DNA-dependent functions, including replication, repair, recombination and gene regulation.The expression of hanA in the heterocysts by using gfp as the reporter gene under control of the hanA promoter is investigated. There was no significant difference in fluorescence in vegetative cells under nitrogen-replete and nitrogen-starvation conditions. And the GFP fluorescence remained clearly present in the heterocysts at 24h and 48h after nitrogen step-down. However, the fluorescence decreased to a low level at 72h and became barely detectable at 96h after nitrogen starvation. The expression of HanA in the heterocysts was further examined by Western blotting, showing that the similar expression decrease from 24h to 96h after nitrogen starvation. Thses results showed that HanA did not play essential roles in heterocysts and thus was gradually degraded in heterocyst. In addition, ectopic expression of hanA in heterocysts did not affect the function and morphology of heterocysts.NtcA is one of the most important transcription factor that globally regulating the expression of a large amount of genes mainly involved in nitrogen metabolism. The expression of many genes that are differentially expressed in heterocysts is also under the control of NtcA. It has been revealed that NtcA acts as DNA binding protein by recognizing a consensus palindromic DNA motif (GTAN8TAC). The hanA promoter region contains three motifs similar to the consensus NtcA-binding sites. They are located to the-278,-83 and-59 upsrteam region from the start codon, respectively. Our EMSA result showed that NtcA could bind to the first and second motifs, with the affinity of the second motif to NtcA is higher than that of the first. These data suggested that NtcA might directly bind to the promoter region of hanA, leading to the decease of hanA transcripts and HanA protein in heterocyst.